1. Field of the Invention
The present invention relates to photolithography of semiconductor devices, and more particularly, to a photolithography system having a frequency domain filter mask.
2. Background Information
Photolithography is a process that is commonly used in the manufacture of integrated circuits. The well-known process involves the deposition of a photoresist layer onto an underlying substrate layer. The photoresist is then selectively exposed to light, which chemically alters the photoresist. The photoresist is then developed and those portions of the photoresist that are exposed to light are either hardened or softened, depending upon whether or not the photoresist is xe2x80x9cnegativexe2x80x9d or xe2x80x9cpositivexe2x80x9d photoresist, respectively.
The pattern that is projected onto the photoresist layer is contained on a mask that is placed within the photolithography exposure tool. The mask is also referred to as a reticle mask. The most common type of photolithography exposure tool is the stepper machine. A mask is placed between the illuminating light and the photoresist. The reticle is typically formed from patterned chromium coated on glass or quartz. The pattern is transferred onto the photoresist by projecting an image of the mask onto the photoresist.
As features on the mask become closer and closer together, diffraction effects begin to appear when the width of the openings on the mask is comparable to the wavelength of the light source. The diffraction effect blurs the light image projected onto the photoresist, resulting in poor resolution. The pattern formed on the photoresist layer will not be an exact replica of the pattern on the reticle mask, producing errors in the manufacturing process. One prior art method of preventing diffraction patterns from interfering with the desired patterning of the photo-resist is to cover selected openings in the mask with a transparent layer that shifts one of the sets of exposing rays out of phase, which will null the interference pattern. This approach is referred to as a phase shift mask.
FIG. 1 shows a prior art phase shift mask. The phase shift mask has parts of the openings in the photoresist layer covered by a phase-shifting layer. This generally requires the deposition of a layer of silicon dioxide onto the mask or reticle and a photomasking process to remove the oxide layer from alternate patterns. Covering every other opening works well for repeated array patterns such as logic and memory products.
Nevertheless, use of the phase shift mask has several disadvantages. First, the design of a phase shift mask is a relatively complicated procedure that requires significant resources. Secondly, because of the nature of a phase shift mask, it is difficult to check whether or not defects are present in the phase shift mask.
Therefore, what is needed is a new method of providing the high resolution capabilities of a phase shift mask using a simpler approach.
A system and method of improving a photolithography process is disclosed. A phase-shift filter is placed between two focusing lenses located between a reticle mask and a wafer. The two focusing lenses combined with the phase-shift filter performs an amplitude and phase adjustment of the mask image in the spatial frequency domain, projecting an image that is equivalent to the differentiation of the light intensity of the mask image onto the wafer, thereby more accurately reproducing the mask pattern on the wafer.